Britain’s High Court is set to rule on whether the government can trigger the U.K.’s exit from the European Union without approval from Parliament. The case, set for a ruling on Thursday, is considered the most important constitutional matter in a generation.

It centers on whether Prime Minister Theresa May can start negotiating Britain’s exit from the EU without a vote in the House of Commons. At the core of the matter is the balance of power between Parliament and the government. Regardless of the victor, the case will likely be immediately appealed to the Supreme Court. May has said she will invoke Article 50 of the EU treaty – triggering two years of official exit talks – by March 31.